The present invention generally relates to bearings and, more particularly, relates to bearings for grinding and milling spindles in machine tools for machining technology.
As is known, machine tools are used for machining components such as metal component. Such machine tools often have a grinding or milling spindle that is driven by a high-speed motor. The work spindle can be connected to an adapter for receiving a work piece or a tool.
Such work spindles of machine tools are guided by means of bearings. These bearings perform many functions. The bearings give the components intended for performing the cutting and advance movements an exact path of movement. The bearings also carry the weight of the spindle shaft and surrounding components and they take up operating forces that arise, without vibrations. Therefore, high rigidity and high damping capacity perpendicular and parallel to the guide path is necessary. In order to obtain a high permanent accuracy when performing grinding and milling movements, there are also demands on the spindle shaft guide means for low frictional forces, low wear, good clearance adjustment capacity and protection from chips, cooling water, dirt and damage.
In practice, high-speed spindles in machining technology work within a range of 20-100,00 revolutions per minute. Such work spindles are guided on conventional ball and roller bearings, magnetic bearings and hydrostatic bearings. Usually the speed characteristics, formed from diameter in mm times speed in r.p.m. (Dxc3x97n), give a greater insight into the prevailing speed conditions. Thus, the range of roller bearings is at a speed characteristic rather below 2.5 million and the range of hydrostatic bearings is rather over 2.5 million.
Ball bearings are less well suited to the working range at high speeds. They are very bulky and so restrict mounting in the machine tool. In order to meet the requirements for low friction at high speed with ball and roller bearings and take up axial and radial forces, often there are several small ball bearings for each taking up an axial or radial force component.
Such ball or roller bearings exhibit punctiform and linear pressure contact with the work spindle. They, therefore, have linear rigidity, which under instantaneous loads can lead to tilting and deflection and other drawbacks resulting therefrom. This impact sensitivity essentially determines the relatively short lifespan of ball and roller bearings. To reduce the impact sensitivity, further stabilizing bearings are provided. All these drawbacks make it uneconomical to use ball and roller bearings with the present-day, ever increasing requirements of the cutting speeds.
Magnetic bearings are capable of use only to a limited extent for application in a milling tool. High forces are generated in milling, which requires a high load-carrying capacity and high radial and axial rigidity of the bearing. A drawback of magnetic bearings is that they are relatively soft and capable of carrying only low loads. Also they are very expensive and elaborate to control only by electronics.
Hydrostatic bearings with a pressure medium of hydraulic oil are used preferably in machine tools which work in the lower speed range. A drawback for use in high-speed machine tools is that the oil, owing to its high viscosity, generates frictional heat and poorly conducts away the frictional heat produced. Also, hydrostatic bearings take up force components in one direction only.
It is an object of the present invention to provide a bearing, for example for use in machine tools for machining technology, which eliminates the above-mentioned drawbacks. In particular the bearing is to have a simple and compact structure and it is to be economical to procure and maintain. The bearing is to be compatible with common methods of the tooling industry and capable of integration.
The invention provides a single bearing for taking up the force components arising. Preferably, the bearing is formed in one piece and takes up axial and/or radial force components that arise on a shaft and adjoining elements. This is a departure from known constructions where several separate bearings are used for such purposes.
By using a single bearing, the movement of the shaft in the bearing can be compared with a gyratory movement, The center of the shaft in the region of the bearing forms the supporting point of the gyroscope. This is why gyroscopic equations can be used to describe the movement and to allow for precession and nutation. As a result of movement analyses, the bearing is provided with specially arranged bearing regions that effectively compensate for precession and nutation of the shaft.
By using a single, preferably one-piece bearing, a compact design is made possible. This in turn allows use at high-speed characteristics of  greater than 2.5 million. The present invention can thus be used in a high-speed machine tool for machining technology, advantageously with direct power transmission via only one flange of the shaft to a machine tool.
The bearing is advantageously hydrostatic and runs on water. Thus, the present invention makes it possible to prestress the bearing with turbulently flowing water which flows through jets into a bearing gap between shaft and bearing, in the inoperative state of the shaft. The shaft is mounted with areal rigidity by means of the film of water. The bearing gap allows compensation of the external forces acting on the spindle shaft, with prestressing of the bearing unit. This, therefore, allows hydrostatically controlled compensation of the forces applied to the bearing from the outside. In particular, the gap width can be controlled by compartment pressure in a simple and uncomplicated manner, without disturbing resonances and/or an increase in amplitude of the shaft occurring during operation.